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TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581

full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) init’s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver. point-to-point:

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TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581

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  1. full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) init’s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver point-to-point: one sender, one receiver reliable, in-order byte steam: no “message boundaries” pipelined: TCP congestion and flow control set window size send & receive buffers TCP: OverviewRFCs: 793, 1122, 1323, 2018, 2581 3: Transport Layer

  2. Roadmap • TCP header and segment format • Connection establishment and termination • Normal Data flow • Retransmission 3: Transport Layer

  3. 32 bits source port # dest port # sequence number acknowledgement number head len not used rcvr window size U A P R S F checksum ptr urgent data Options (variable length) application data (variable length) TCP segment structure URG: urgent data (generally not used) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data now (generally not used) # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) 3: Transport Layer

  4. TCP Headers: like UDP? • Source and destination port numbers • Checksum • Data length? Rely of length in IP header? 3: Transport Layer

  5. TCP Headers: Familiar? • Sequence Number field ( 32 bit) • Sequence Number field indicates number of first byte in the packet • Receiver Window Size (16 bit) • Window like for GBN or selective repeat, but window size not fixed – variable based on receiver feedback • Acknowledgment Field (32 bit) • The acknowledgement field contains the next sequence number it is expecting thus implicitly acknowledging all previous segments. • Cumulative acks not selective acks or negative acks 3: Transport Layer

  6. Seq. #’s: byte stream “number” of first byte in segment’s data ACKs: seq # of next byte expected from other side cumulative ACK Q: how receiver handles out-of-order segments A: TCP spec doesn’t say, - up to implementor time TCP seq. #’s and ACKs Host B Host A User types ‘C’ Seq=42, ACK=79, data = ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ Seq=79, ACK=43, data = ‘C’ host ACKs receipt of echoed ‘C’ Seq=43, ACK=80 simple telnet scenario 3: Transport Layer

  7. Implications of Field Length • 32 bits for sequence number (and acknowledgement) ; 16 bits for advertised window size • Implication for maximum window size? Window size <= ½ SequenceNumberSpace • Requirement easily satisfied because receiver advertised window field is 16 bits • 232 >> 2* 216 • no wrap-around in maximum segment lifetime (MSL) = 120 seconds (?) 3: Transport Layer

  8. Implications of Field Length (cont) • Advertised Window is 16 bit field => maximum window is 64 KB • Is this enough to fill the pipeline? • Pipeline = delay*BW product • 100 ms roundtrip and 100 Mbps => 1.19 MB • Sequence Number is 32 bit field => 4 GB • With a MSL of 120 seconds will this ever wrap too soon? • 4 GB/120 sec = 273 Mbps • Gigabit Ethernet? STS-12 at 622 Mbps? 3: Transport Layer

  9. TCP Header: Flags (6 bits) • Connection establishment/termination • SYN – establish; sequence number field contains valid initial sequence number • FIN - terminate • RESET - abort connection because one side received something unexpected • PUSH - sender invoked push to send • URG – indicated urgent pointer field is valid; special data - record boundary • ACK - indicates Acknowledgement field is valid 3: Transport Layer

  10. TCP Header: ACK flag • ACK flag – if on then acknowledgement field valid • Once connection established no reason to turn off • Acknowledgment field is always in header so acknowledgements are free to send along with data 3: Transport Layer

  11. TCP Header: URG • If URG flag on, then URG pointer contains a positive offset to be added to the sequence number field to indicate the last byte of urgent data • No way to tell where urgent data starts – left to application • TCP layer informs receiving process that there is urgent data 3: Transport Layer

  12. Out-of-band data • URG is not really out-of-band data; Receiver must continue to read byte stream till reach end of urgent data • If multiple urgent segments received, first urgent mark is lost; just one urgent pointer • How to get out-of-band data if need it? • Seperate TCP connection 3: Transport Layer

  13. URG • How helpful is this? • Telnet and Rlogin use URG when user types the interrupt key; FTP uses when user aborts a file transfer • Is this worth a whole header field and a flag? • Doesn’t help that implementations vary in how they interpret the urgent pointer (point to last byte in urgent data or byte just past the last byte of urgent data) 3: Transport Layer

  14. TCP Header: PSH • Intention: use to indicate not to leave the data in a TCP buffer waiting for more data before it is sent • In practice, programming interface rarely allows application to specify • Instead TCP will set if this segment used all the data in its send buffer • Receiver is supposed to interpret as deliver to application immediately; most TCP/IP implementations don’t delay deliver in the first place 3: Transport Layer

  15. TCP Header: Data boundaries? • In general with UDP, application write of X bytes data results in a UDP datagram with X bytes of data – not so with TCP • In TCP, the stream of bytes coming from an application is broken at arbitrary points into the “best” size chunks to send • Sender may write 10 bytes then 15 then 30 but this is not in general visible to the receiver 3: Transport Layer

  16. Record Boundaries • Could try to use URG and PSH to indicate record boundaries • socket interface does not notify app that push bit or urgent bit is on though! • In need record boundaries, applications must always insert their own by indicating it in the data (ie. Data is record len + record format) 3: Transport Layer

  17. TCP Header: Header Length • Header Length (4 bits) • needed because options field make header variable length • Expressed in number of 32 bit words • 4 bits field => 4*16 = 60 bytes; 20 bytes of normal gives 40 bytes possible of options 3: Transport Layer

  18. TCP Header: Common Options • Maximum Segment Size Option can be set in SYN packets • Options used to extend and test TCP • Each option is: • 1 byte of option kind • 1 byte of option length (except for kind = 0 for end of options and kind =1 for no operation) • Other options • window scale factor: if don’t want to be limited to 216 bytes in receiver advertised window) • timestamp option: if 32 bit sequence number space will wrap in MSL; add 32 bit timestamp to distinguish between two segments with the same sequence number 3: Transport Layer

  19. Recall:TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) client: connection initiator Socket clientSocket = new Socket("hostname","port number"); server: contacted by client Socket connectionSocket = welcomeSocket.accept(); Three way handshake: Step 1:client end system sends TCP SYN control segment to server specifies initial seq # Step 2:server end system receives SYN, replies with SYNACK control segment ACKs received SYN allocates buffers specifies server-> receiver initial seq. # Step 3: client acknowledges servers initial seq. # TCP Connection Management 3: Transport Layer

  20. Active participant Passive participant (client) (server) SYN, SequenceNum = x , y 1 + SYN + ACK, SequenceNum = x Acknowledgment = ACK, Acknowledgment = y + 1 Three-Way Handshake Note: SYNs take up a sequence number even though no data bytes 3: Transport Layer

  21. Connection Establishment • Both data channels opened at once • Three-way handshake used to agree on a set of parameters for this communication channel • Initial sequence number for both sides • Receiver advertised window size for both sides • Optionally, Maximum Segment Size (MSS) for each side; if not specified MSS of 536 bytes is assumed to fit into 576 byte datagram 3: Transport Layer

  22. Initial Sequence Numbers • Chosen at random in the sequence number space? • Well not really randomly; intention of RFC is for initial sequence numbers to change over time • 32 bit counter incrementing every 4 microseconds • Vary initial sequence number to avoid packets that are delayed in network from being delivered later and interpreted as a part of a newly established connection 3: Transport Layer

  23. Special Case: Timeout of SYN • Client will send three SYN messages • Increasing amount of time between them (ex. 5.8 seconds after first, 24 seconds after second) • If now responding SYNACK will terminate 3: Transport Layer

  24. Special Case: Simultaneous active SYNs • Possible but improbable for two ends to generate SYNs for the same connection at the same time • SYNs cross in the network • Both reply with SYNACK and connection is established 3: Transport Layer

  25. Connection Termination • Each side of the bi-directional connection may be closed independently • 4 messages: FIN message and ACK of that FIN in each direction • Each side closes the data channel it can send on • One side can be closed and data can continue to flow in the other direction, but not usually • FINs consume sequence numbers like SYNs 3: Transport Layer

  26. Closing a connection: client closes socket:clientSocket.close(); Step 1:client end system sends TCP FIN control segment to server Step 2:server receives FIN, replies with ACK. Closes connection, sends FIN. client server close FIN ACK close FIN ACK timed wait closed TCP Connection Management (cont.) 3: Transport Layer

  27. Step 3:client receives FIN, replies with ACK. Enters “timed wait” - will respond with ACK to received FINs Step 4:server, receives ACK. Connection closed. Note:with small modification, can handly simultaneous FINs. TCP Connection Management (cont.) client server closing FIN ACK closing FIN ACK timed wait closed closed 3: Transport Layer

  28. TCP Connection Management (cont) TCP server lifecycle TCP client lifecycle 3: Transport Layer

  29. Typical Client Transitions CLOSED Active open /SYN Typical Server Transitions Passive open Close Close LISTEN /SYN + ACK Send/ SYN SYN/ SYN/SYN + ACK SYN_RCVD SYN_SENT ACK SYN+ACK/ /ACK ESTABLISHED: data transfer! Close /FIN Close /FIN FIN/ACK FIN_WAIT_1 CLOSE_WAIT FIN/ACK ACK Close /FIN ACK + FIN/ACK FIN_WAIT_2 CLOSING LAST_ACK Timeout after two ACK ACK segment lifetimes FIN/ACK TIME_WAIT CLOSED 3: Transport Layer

  30. Netstat • netstat –a –n • Shows open connections in various states • Example: Active Connections Proto LocalAddr ForeignAddr State TCP 0.0.0.0:23 0.0.0.0:0 LISTENING TCP 192.168.0.100:139 207.200.89.225:80 CLOSE_WAIT TCP 192.168.0.100:1275 128.32.44.96:22 ESTABLISHED UDP 127.0.0.1:1070 *:* 3: Transport Layer

  31. Time Wait State • Wait 2 times Maximum Segment Lifetime (2 MSL) • Provides protection against delayed segments from an earlier incarnation of a connection being interpreted as for a new connection • Maximum time segment can exist in the network before being discarded • Time-To-Live field in IP is expressed in terms of hops not time • TCP estimates it as 2 minutes • During this time, combination of client IP and port, server IP and port cannot be reused • Some implementations say local port cannot be reused while it is involved in time wait state 3: Transport Layer

  32. RST • RST flag • Abortive release of a connection rather than the orderly release with FINs • We saw client browser ended its connections that way - not good form 3: Transport Layer

  33. Data (SequenceNum) Sender Receiver Acknowledgment + AdvertisedWindow Data Transfer (Simplified One-Way) 3: Transport Layer

  34. TCP Sender: Simplified State Machine event: data received from application above simplified sender, assuming • one way data transfer • no flow, congestion control create, send segment wait for event event: timer timeout for segment with seq # y wait for event retransmit segment event: ACK received, with ACK # y ACK processing 3: Transport Layer

  35. TCP Sender: Simplified Pseudo-code 00sendbase = initial_sequence number 01 nextseqnum = initial_sequence number 02 03 loop (forever) { 04 switch(event) 05 event: data received from application above 06 create TCP segment with sequence number nextseqnum 07 start timer for segment nextseqnum 08 pass segment to IP 09 nextseqnum = nextseqnum + length(data) 10 event: timer timeout for segment with sequence number y 11 retransmit segment with sequence number y 12 compue new timeout interval for segment y 13 restart timer for sequence number y 14 event: ACK received, with ACK field value of y 15 if (y > sendbase) { /* cumulative ACK of all data up to y */ 16 cancel all timers for segments with sequence numbers < y 17 sendbase = y 18 } 19 else { /* a duplicate ACK for already ACKed segment */ 20 increment number of duplicate ACKs received for y 21 if (number of duplicate ACKS received for y == 3) { 22 /* TCP fast retransmit */ 23 resend segment with sequence number y 24 restart timer for segment y 25 } 26 } /* end of loop forever */ Simplified TCP sender 3: Transport Layer

  36. TCP Receiver: ACK generation[RFC 1122, RFC 2581] TCP Receiver action delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK immediately send single cumulative ACK send duplicate ACK, indicating seq. # of next expected byte (sender can use as hint of selective repeat) immediate ACK if segment starts at lower end of gap Event in-order segment arrival, no gaps, everything else already ACKed in-order segment arrival, no gaps, one delayed ACK pending out-of-order segment arrival higher-than-expect seq. # gap detected arrival of segment that partially or completely fills gap 3: Transport Layer

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